The present invention relates to an abrading method and abrading machine suitable for abrading liquid crystal cells for liquid crystal displays.
In recent years, rectangular thin glass substrates 0.5 mm to 1.5 mm thick have been used for flat displays such as liquid crystal displays. Precise flatness is required of these substrates. Glass substrates molded by standard industrial processes contain minute surface imperfections, waviness and irregularities. Such substrates are not flat enough to be used in flat displays without further processing. Generally, therefore, the surfaces of molded glass substrates are abraded to a desired flatness on an abrading machine.
In a liquid crystal display, a liquid crystal material is encapsulated between a pair of glass substrates forming a liquid crystal cell. One of the glass substrates is a color filter substrate while the other is a TFT (Thin Film Transistor) array substrate. Liquid crystal cells tend to become increasingly thin as the weight of liquid crystal displays is reduced. In order to obtain a thin liquid crystal cell, the liquid crystal cell is typically abraded by using a double-sided abrading machine until the color filter substrate and the TFT array substrate are laminated, that is, abraded to a desired thickness after forming the liquid crystal cell.
A conventional double-sided abrading machine will be described below with reference to FIGS. 4, 5A and 5B. FIG. 4 is a side view of a conventional double-sided abrading machine. FIGS. 5A and 5B show a work carrier used in a conventional double-sided abrading machine, FIG. 5A is a plan view of the work carrier, and FIG. 5B is a sectional view taken along line Axe2x80x94A in FIG. 5A. FIG. 4 also shows an abrasion process in which a liquid crystal cell 6 as a workpiece is being abraded by using two work carriers of the type shown in FIG. 5.
The double-sided abrading machine 100 shown in FIG. 4 includes an upper plate 4 and a vertically opposed lower plate 5, as well as a carrier 101 serving as a holding member placed between the upper plate 4 and the lower plate 5.
A shaft 16 is secured to the upper plate 4 and a shaft 17 is secured similarly to the lower plate 5. The shaft 16 and the shaft 17 are rotated by a drive means (not shown) to rotate the upper plate 4 and the lower plate 5.
As shown in FIGS. 5A and 5B, a holding hole 101a is formed in the carrier 101 to fit the outer shape of the liquid crystal cell 6, which is inserted and held in holding hole 101a. The hole is slightly larger the liquid crystal cell 6 to make it easy to insert liquid crystal cell 6. The liquid crystal cell has a TFT array substrate 61 and a color filter substrate 62 stacked together, above and below a thin liquid crystal layer (not shown). The color filter substrate 62 has a surface area slightly smaller than that of the TFT array.
The double-sided abrading machine 100 has a small-diameter sun gear 7 inside the perimeter of the upper plate 4 and lower plate 5, and a large-diameter internal gear 8 around the perimeter. The sun gear 7 is secured to a drive shaft (not shown) which passes through the lower plate 5. Gear 7 rotates coaxially with the shaft 17 as the drive shaft rotates. The internal gear 8 is mounted outside the upper plate 4 and lower plate 5 and driven coaxially with the shaft 17 by a drive source (not shown).
The carrier 101 has a gear formed along its circumference, as shown in FIG. 5A. This gear is positioned so as to mesh with the sun gear 7 and internal gear 8. Therefore, since it is held between the rotating upper plate 4 and lower plate 5 and meshed with the sun gear 7 and internal gear 8, the carrier 101 revolves around the sun gear 7 while rotating on its axis. The front and rear faces of the liquid crystal cell 6, held by the carrier 101 and pressed by the upper plate 4 and lower plate 5, are abraded while an abrasive is supplied automatically between the upper plate 4 and lower plate 5.
In a conventional arrangement, a workpiece is held by a single carrier throughout the entire process of mechanical abrasion. Consequently, when a large volume of stock is removed, the clearance between the upper plate and carrier (that is, the range of the carrier""s vertical displacement) increases, making the workpiece prone to breakage and the like, as detailed below.
The thickness of the carrier 101 must be smaller than the target thickness of the workpiece (that is, the liquid crystal cell 6) after mechanical abrasion. Otherwise, the upper plate 4 and lower plate 5, between which the carrier 101 is placed to receive and hold the liquid crystal cell 6, cannot come into contact with the liquid crystal cell 6, which in turn makes it impossible to grind the liquid crystal cell 6. Consequently, a problem arises if a large volume of stock is to be removed, as is the case with a liquid crystal cell including laminated substrates; for example, the TFT array substrate 61 and the color filter substrate 62 with a smaller surface area than the TFT array (see FIG. 4). Specifically, with such a workpiece, there is a large clearance L between the upper plate 4 and carrier 101 in an early stage of mechanical abrasion. This may cause carrier 101 to contact an exposed portion of the top surface of TFT array substrate 61, resulting in chips, cracks, or other damage to the liquid crystal cell 6. Damage is noticeable especially when the liquid crystal cell 6 is abraded on the conventional double-sided abrading machine 100 compared to other types of workpieces.
FIG. 6 shows how the liquid crystal cell 6 is abraded on the conventional double-sided abrading machine 100. As shown in FIG. 6, the liquid crystal cell 6 consists of the TFT array substrate 61 and color filter substrate 62. Normally, the TFT array substrate 61 and color filter substrate 62 have the same thickness, but the TFT array substrate 61 generally has a larger surface area than the color filter substrate 62 to secure space for electrode wires. Therefore, a step is formed around the liquid crystal cell 6. If the liquid crystal cell 6 with such a step is abraded on the conventional double-sided abrading machine 100, it is often the case not only that the carrier 101 is displaced vertically during mechanical abrasion as shown in FIG. 6A, but also that the carrier 101 contacts the step around the liquid crystal cell 6 as shown in FIG. 6B, resulting in breakage of the carrier 101. If the thickness of the carrier 101 is increased these problems will not arise, but the desired stock removal then cannot be performed.
It is an object of the present invention to provide an abrading method and abrading machine which will allow a workpiece to be abraded without damage even in the case of heavy stock removal. This is done by stacking a plurality of carriers for holding the workpiece, so that the carrier height may be varied according to the stage of mechanical abrasion. In this way, the vertical displacement of the carriers can be restricted and breakage of workpieces caused by carriers can be avoided, even in the case of heavy stock removal, as long as the clearance between the upper plate and carriers is kept within a designated range.
In accordance with a first aspect of the present invention, a work holding member for mechanical abrasion is provided, comprising a first holding member which has a socket for receiving a workpiece and transmits external drive force to the workpiece, and a second holding member which is placed on the first holding member and has a socket for receiving the workpiece.
Since the work holding member for mechanical abrasion according to the present invention includes a first holding member and second holding member, the workpiece can be held by the first holding member alone (by taking away the second holding member), depending on the abraded quantity. Specifically, the workpiece can be held by the first and second holding members in an early stage of mechanical abrasion, while in the last stage of mechanical abrasion the workpiece can be abraded to a target thickness using only the first holding member. This makes it possible to restrict vertical displacement of the work holding member for mechanical abrasion even in the case of heavy stock removal.
Since the first holding member according to the present invention has the function of transmitting external drive force, it corresponds to the carrier 101 described above. On the other hand, the second holding member has the function of a spacer which limits the clearance between the upper plate and carriers to a designated value. The second holding member does not need to have the function of transmitting the external drive force to the workpiece. Alternatively, the external drive force may be transmitted to the workpiece through the second holding member. As another alternative, additional holding members may be used.
If the workpiece is a laminate of two plates such as a liquid crystal cell, the color filter substrate and TFT array substrate are generally equal in thickness and thus the color filter substrate and TFT array substrate are abraded from above and below, respectively, by an equal amount on a double-sided abrading machine. Consequently, the target thickness of the liquid crystal cell as a whole after mechanical abrasion is twice the target thickness of the color filter substrate or TFT array substrate after mechanical abrasion. In this way, when the workpiece is a laminate of two plates, the thickness of the first holding member should be equal to or larger than the target thickness of a single plate (either the color filter substrate or TFT array substrate in the case of a liquid crystal cell) after mechanical abrasion. Incidentally, although there is a liquid crystal layer between the color filter substrate and TFT array substrate, its thickness is negligible compared to the thicknesses of the color filter substrate and TFT array substrate. Accordingly, the liquid crystal layer is ignored herein.
In the work holding member for mechanical abrasion according the present invention, if the thicknesses of the workpiece before and after mechanical abrasion are denoted by T1 and T2, respectively, and the thicknesses of the first holding member and second holding member are denoted by t1 and t2, respectively, the following relations should be satisfied:
T1 greater than t1+t2
(xc2xd)T2xe2x89xa6t1
Furthermore, in the work holding member for mechanical abrasion according the present invention, if a workpiece having two laminated substrates equal in thickness (such as a liquid crystal cell) is abraded, the thicknesses of the first holding member and second holding can be determined as follows. If the thicknesses of the workpiece before and after mechanical abrasion are denoted by T1 and T2, respectively, and the thicknesses of the first holding member and second holding member are denoted by t1 and t2, respectively, the following relations should be satisfied:
T1 greater than t1+t2 greater than T2
(xc2xd)T1xe2x89xa7t1 greater than (xc2xd)T2
T2 greater than t1
According to a second aspect of the present invention, an abrading method is provided which includes placing a workpiece, held by a holding member, between a pair of upper and lower plates and abrading the workpiece by rotating the pair of plates and the holding member. This method includes the following steps: a first abrading step of abrading the workpiece held by first and second holding members disposed in the direction of the thickness of the workpiece, removing the first or second holding member after a predetermined quantity is abraded from the workpiece, and a second abrading step of further abrading the workpiece.
The abrading method of the present invention grinds the workpiece using the first and second holding members until the volume of material removed reaches a designated value. Therefore, the clearance between the holding members and upper plate can be reduced even if a large volume of stock is to be removed. Moreover, since the first or second holding member is taken away as the workpiece becomes thin, subsequent abrading operations are not hindered. Thus, according to the abrading method of the present invention, whether the volume of stock removed from the workpiece has reached a designated value can be judged based on the clearance between the first or second holding member which is not removed in the second abrading step and the upper plate.
In addition, according to the abrading method of the present invention, the workpiece may have a step around the perimeter thereof and the first abrading process may be performed with either the first or second holding member mounted on the step. Workpieces with such a step include liquid crystal cells.
As described above, the second holding member of the work holding member for mechanical abrasion according to the present invention functions as a spacer, and has the function of restricting vertical displacement of the first holding member (the first holding member corresponding in this case to a conventional carrier). Therefore, according to another aspect of the present invention, an abrading method is provided which includes placing a workpiece, held by a work carrier, between a pair of upper and lower plates and abrading the workpiece by rotating the pair of plates and the work carrier. This method comprises a first abrading step of abrading the workpiece by restricting vertical displacement of the work carrier; a step of lifting this restriction on the vertical displacement of the work carrier after a predetermined quantity is abraded from the workpiece, and a second abrading step of further abrading the workpiece.
In the abrading method of the present invention, it is desirable that in the second abrading step, the clearance between the work carrier and upper plate should be equal to or less than the thickness of the work carrier.
According to a further aspect of the present invention, an apparatus is provided for performing the abrading method of the present invention. Specifically, an abrading machine according to the present invention includes a lower plate for mounting a workpiece; an upper plate disposed opposite the lower plate, a holding member for holding a workpiece disposed between the lower plate and the upper plate and providing driving force to the workpiece; and a spacer disposed between the holding member and the upper plate and driven along with the workpiece. The abrading machine of the present invention makes it possible to grind a workpiece to a target thickness, keeping the clearance between the upper plate and carriers within a designated range even in the case of heavy stock removal.